Reply by Joseph Gwinn March 16, 20182018-03-16
On Mar 15, 2018, Jasen Betts wrote
(in article <p8d2ah$mp0$2@gonzo.alcatraz>):

> On 2018-03-14, Steve Wilson<no@spam.com> wrote: > > Jeroen Belleman<jeroen@nospam.please> wrote: > > > > > Nothing prevents the use of coupling transformers at the end(s) of > > > a length of coax. > > > > No, that doesn't work. It allows return currents to flow on the outside of > > the shield. The makes the cable radiate RFI. > > How's that going to happen? > > As I see it with the coax isolated the only current flowing in the shield > is the return of the current flowing in the core and that current will be > drawn to the inside of the shield by the magnetic field of the core.
Yes. We use RF transformers to break ground loops in coax all the time. If it&rsquo;s really critical, one uses shielded transformers, but if it&rsquo;s low power, a transformer the size of a peppercorn is too small to radiate much.
> > To prevent it, the coax must be > > grounded to the case. This can provoke ground loops. > > that's pretty much guaranteed to cause ground loops.
Yep. For RF, what also works is to make the inputs to power-frequency ground loops. See MIL-STD-461 method CS-109. Joe Gwinn
Reply by Steve Wilson March 16, 20182018-03-16
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:

> On Mon, 12 Mar 2018 12:56:00 GMT, Steve Wilson <no@spam.com> wrote:
>>What are your thoughts?
> Unrelated to this request, but I ran across this from HP on using RJ45 > connectors for analog instrumentation applications. They ended up > using it for strain gauges, which, as most of you know, are rather low > level devices.
> http://www.vtiinstruments.com/Catalog/Technotes/RJ-45_Excels_For_Stria_Ga > ge_Connection.pdf
> The combination of good electrical performance and cheap/reliable > termination of shielded or unshielded twisted pairs is useful.
> --sp
Very interesting information. The connector is much more reliable than I thought. Excellent stability, 750 mating cycles, inexpensive and available everywhere. Pretty hard to beat. Directly applicable to my request. Thanks, Speff
Reply by Les Cargill March 15, 20182018-03-15
Jeff Liebermann wrote:
> On Wed, 14 Mar 2018 18:20:45 -0500, Les Cargill > <lcargill99@comcast.com> wrote: > >> There's also something to be said for M12 on the outside of boxes. > > Yep: > <http://www.iebmedia.com/index.php?id=5873&parentid=63&themeid=255&showdetail=true> > >> They're harder to kill and make positive contact. You can have a >> throughhole pigtail like you'd have using the above object. > > They're also substanitally more expensive than RJ45. >
Very much so! -- Les Cargill
Reply by Spehro Pefhany March 15, 20182018-03-15
On Mon, 12 Mar 2018 12:56:00 GMT, Steve Wilson <no@spam.com> wrote:

> >What are your thoughts?
Unrelated to this request, but I ran across this from HP on using RJ45 connectors for analog instrumentation applications. They ended up using it for strain gauges, which, as most of you know, are rather low level devices. http://www.vtiinstruments.com/Catalog/Technotes/RJ-45_Excels_For_Stria_Gage_Connection.pdf The combination of good electrical performance and cheap/reliable termination of shielded or unshielded twisted pairs is useful. --sp
Reply by Jasen Betts March 15, 20182018-03-15
On 2018-03-14, Steve Wilson <no@spam.com> wrote:
> Jeroen Belleman <jeroen@nospam.please> wrote: > >> Nothing prevents the use of coupling transformers at the end(s) of >> a length of coax. > > No, that doesn't work. It allows return currents to flow on the outside of > the shield. The makes the cable radiate RFI.
How's that going to happen? As I see it with the coax isolated the only current flowing in the shield is the return of the current flowing in the core and that current will be drawn to the inside of the shield by the magnetic field of the core.
> To prevent it, the coax must be > grounded to the case. This can provoke ground loops.
that's pretty much guaranteed to cause ground loops. -- This email has not been checked by half-arsed antivirus software
Reply by Phil Hobbs March 14, 20182018-03-14
On 03/12/2018 03:56 PM, Steve Wilson wrote:
> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> What do you actually want to accomplish? I's be very unlikely to use an >> RJ-45 for anything but Ethernet, myself. Most newer instruments have >> Ethernet connectivity out of the box, and older ones can be controlled >> via e.g. a Prologix GPIB-Ethernet box. > >> Cheers > >> Phil Hobbs > > I have the Prologix interfaces. I do not want to use GPIB, RS-232, USB, HDMI, > or any of the other interfaces that have been discussed. > > Say I need to monitor a hypothetical process and measure temperature, > pressure, flow, etc. I need to make the necessary sensors and connect them to > a controller to gather the data and send it back to a host computer. The > controller is connected via the LAN. > > Say I need a bidirectional interface to control the sensors. To connect the > individual sensors to the controller, I propose using the same RJ-45 > interface used on the LAN, but using my own protocol instead of ethernet. > > Why would you not use RJ-45? >
For one reason, everybody assumes that an RJ45 is an Ethernet connector, so care is needed so that nothing gets blown up. For another reason, they aren't the most reliable connectors in captivity, especially when daisy-chained. For a third, there are a lot of different styles of Ethernet patch cords out there--especially crossover cords--and some squaddy is bound to use the wrong one. You'll get blamed for the resulting failure, as sure as God made little green apples. I've often repurposed video cables (Displayport and HDMI) but not RJ45, thanks. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by Jeff Liebermann March 14, 20182018-03-14
On Wed, 14 Mar 2018 18:20:45 -0500, Les Cargill
<lcargill99@comcast.com> wrote:

>There's also something to be said for M12 on the outside of boxes.
Yep: <http://www.iebmedia.com/index.php?id=5873&parentid=63&themeid=255&showdetail=true>
>They're harder to kill and make positive contact. You can have a >throughhole pigtail like you'd have using the above object.
They're also substanitally more expensive than RJ45. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Reply by Les Cargill March 14, 20182018-03-14
Jeff Liebermann wrote:
> On Tue, 13 Mar 2018 02:45:04 GMT, Steve Wilson <no@spam.com> wrote: >
<snip>
> > Besides users that cram a USB connector into the ethernet jack, > there's also the problem of the gold wire connections being mangled > inside the RJ45 jack. Three ways that I know of to do that: > 1. Cram something into the RJ45 that doesn't belong and wiggle. > 2. Use an RJ11 or RJ14 plug in an RJ45 jack. (More on this below) > 3. Cram an RJ45 plug into the jack which was NOT crimped. > The result of all of these is to have the gold wires in the jack move > over a notch, thus shorting two adjacent pins together. If the > adjacent pin carries power, it might blow something up. Internally, > the jack has a plastic "comb" that acts as a separator and spreader > for the gold wires. It doesn't take much to lift a gold wire from the > comb, move it to the adjacent slot, and produce a short. > > The mangled RJ45 has become common enough that designers are defending > themselves by making the connector an easily replaceable part. Most > common is using a dual RJ45 "barrel" connector. Something like this: > <https://www.amazon.com/Waterproof-CAT5e-RJ45-Feed-Thru-Coupler/dp/B00CULLAWE> > If the connector gets mangled, just unplug, remove and replace. >
There's also something to be said for M12 on the outside of boxes. They're harder to kill and make positive contact. You can have a throughhole pigtail like you'd have using the above object.
> Anyway, good luck. > >
-- Les Cargill
Reply by whit3rd March 14, 20182018-03-14
On Wednesday, March 14, 2018 at 3:36:03 AM UTC-7, Steve Wilson wrote:
> Jeroen Belleman <jeroen@nospam.please> wrote: > In any case, I'm worried about RJ-45 cable noise affecting nearby sensitive > circuits. So far, I haven't seen any problems from the LAN even when > unshielded, but local FM stations can be a problem.
> > Nothing prevents the use of coupling transformers at the end(s) of > > a length of coax. > > I need to tranfer power. That is done in RJ-45 using poe. Coax would > require a separate cable and connector. > > I also need bidirectional data transfer. That means another coax. All these > cables and connectors would take time to connect and disconnect. Two > identical coax cables could be mistakenly swapped. RJ-45 puts it all in one > connector.
There's plenty of connectors out there: the usual S-video connector is dual coax with an optional overall shield. You can do power POE-style over the two braids, and high bandwidth signals on the center conductors, using the usual Ethernet-like magnetics. It ain't high performance coax, but it's attractively off-the-shelf at low cost.
Reply by Steve Wilson March 14, 20182018-03-14
Steve Wilson <no@spam.com> wrote:

> There are about as many ways to measure shielding effectiveness as there > are people making the measurements!
Here's how I measured shielding effectiveness. 1. get two female 50 ohm BNC terminators, such as https://www.pasternack.com/0.5-watts-bnc-female-rf-load-up-to-1-ghz-pe6027- 50-p.aspx 2. get two identical 50 ohm coax cables of a suitable length. 3. connect one coax to the signal generator and terminate in 50 ohms. 4. connect the other coax to a spectrum analyzer. Terminate the free end with a 50 ohm terminator. 5. run the two coax cables together side by side. I tried various ways to change the coupling. Twisting the cables together with a slow twist seemed to produce the best and most repeatable results. 6. The spectrum analyzer usually showed the crosstalk about 80 db down from the signal generator output using ordinary cheap coax. This method simulates the problem of running coax cables side-by-side, such as a signal generator driving an amplifier, with the output going through another coax to a spectrum analyzer or network analyzer. The coax should be high quality double shielded or hardline.